A wide variety of containers are used to transport bulk materials. Typically, containers for transporting bulk materials are constructed of sheet metal either fashioned into a tank or an open container such as a fixed truck box, pivoting dump truck box, or roll-off container. As used in connection with the present invention, "containers" includes open containers such as pivoting dump truck boxes, fixed truck boxes, roll-off containers detachable from a truck, intermodal containers designed to be transported by ship, train or truck, substantially enclosed tanks for use in transporting bulk liquids, and any other container assembly for transporting bulk materials that is adapted to be attached to a truck, truck-like vehicle, or trailer. Containers within the meaning of that term as used in connection with the present invention are designed to transport bulk materials in loads ranging from one-half ton or more. The materials can include soil, rocks, grains, foods, liquids, etc.
The containers are typically manufactured using sheet metal (typically steel) attached to a framework of structural members. Sheet metal is used to manufacture the boxes because of its strength and resistance to wear--both properties that are required during typical use of the trucks. In spite of the durability and toughness provided by the sheet metal boxes, there are drawbacks associated with metal truck boxes and other containers.
One problem is that the truck boxes and other containers manufactured with sheet metals are themselves typically relatively heavy which limits the loads that can be transported by the trucks. Furthermore, the sheet metal used to manufacture the truck boxes and containers typically corrodes easily, reducing the useful life of the containers. In addition, it may not be desirable or allowable to allow some materials to come into contact with the sheet metal, thereby requiring the use of expensive liners or other methods to prevent contact between the materials and the sheet metal.
Furthermore, the sheet metal can make unloading difficult in some situations. For example, in colder climates moisture in the materials may cause them to freeze to the interior of the container, typically requiring hand labor to dislodge the material. In some situations, the entire container may need to be heated to facilitate unloading and/or pressurized water may be required to completely unload the container. All of these actions raise the cost of transporting materials using conventional containers manufactured from sheet metal.
In addition to freezing, friction between the materials and the sheet metals used in the containers can also make unloading difficult, requiring the operator, in some situations, to rapidly accelerate and decelerate the containers (which may be mounted to pivot about the end of the truck) in an attempt to jar the materials loose. Such actions add unnecessary stress and wear to the containers as well as the hydraulic lifts used to pivot them, adding the cost to repair any damage and potentially reducing the useful life of the containers and/or hydraulic lifts.
In addition to causing difficulty in unloading when the load remains in the container, such situations may also result in personal injury or damage to the truck because the probability of, for example, a dump truck tipping over is greatly increased when its box is raised to dump a load of material. The probability of tipping is increased because the load typically stays in the front end of the box (which is raised the highest). The raised load may raise the center of gravity of the truck above a point at which the truck is stable, particularly in locations, such as construction sites, where the ground is uneven or unstable.
To address the issue of friction between the load and truck box, some users may coat the interior of the truck box with a release agent, such as diesel fuel, that assist in removal of the load during dumping. That practice can cause undesirable environmental contamination as the materials unloaded from the container invariably carry with them some of the diesel fuel or other release agent.